The present invention relates to apparatus and techniques for treating secondary waste water effluent and more particularly to the treatment of such effluent with ultraviolet radiation.
In order to minimize the health hazards associated with the disposal of sewage, it is necessary to adequately disinfect the water before it is fed back into rivers and streams. Conventionally, disinfecting has been done by a subsequent chlorination step; but chlorination leaves residual free chlorine in the effluent and also forms potentially carcinogenic chlorinated hydrocarbons, both of which are unacceptable under present health and ecological standards. While removal of the residual chlorine and some chlorinated compounds is feasible, this additional step substantially increases the overall cost of the sewage treatment. Accordingly alternative purification methods have been sought.
Ozone treatment has been proposed as one alternative of chlorination, but the cost involved is quite prohibitive.
Iodine, bromine and bromine chloride processes are also being tested, but no data for large scale operations is yet available.
It has also long been known that ultraviolet radiation has a pronounced lethal effect on microorganisms and ultraviolet lamps, which emit ultraviolet rays in the range of 2,500 to 2,700 Angstroms, have been used to purify gases and liquids. U.S. Pat. No. 3,182,193 discloses a system for subjecting a fluid to such ultraviolet radiation. However, heretofore, suspended solids and high fluid volume of secondary sewage treatment have prohibited the effective use of ultraviolet radiation.
I have previously discovered that intensity is more effective in obtaining bacterial kill than long dwell time at low intensity. But intensity drops off hyperbolically at the distance from the source of the radiation increases. Therefore, to assure high intensity purification, my earlier U.S. Pat. No. 3,837,800 discloses my "thin film" concept and a device constructed to cause the inlet fluid to assume a sheet-like flow pattern as it is directed sequentially past a row of ultraviolet lamps. While this patent discloses that several such devices may be stacked to increase the flow rate, such arrangement is not readily adaptable for use in commercial sewage treatment plants which are designed to handle millions of gallons of waste water a day.